Modern commercial jet aircraft have turbofan engines that are started using one or more of a variety of different engine starters. For example, smaller business jets typically include a direct current (DC) motor powered by an on-board battery or a ground power source. While suitable for smaller jets, this arrangement may not be suitable for larger commercial jets because of the high weight associated with batteries large enough to supply starting power for larger turbofan engines.
Another method used for starting larger commercial transport aircraft engines is disclosed in U.S. Pat. No. 4,456,830 to Cronin, and includes using ground-supplied electrical power coupled to an alternating current (AC) starter motor. The AC motor initiates the engine starting process in a synchronous manner that is coordinated with the engine rotation speed. An on-board auxiliary power unit (APU) takes over supplying power to the starter motor after the engine speed achieves five percent to ten percent of the high pressure rotor speed. After the engine speed further increases, fuel is injected into the engine combustion chamber and is ignited in a “light off” process. The starter is subsequently cut out.
The foregoing methods are directed primarily to starting an aircraft engine while the aircraft is on the ground. However, in some cases, the engine must be restarted in flight. Most existing arrangements for in-flight restart rely on the engine “windmilling” (i.e., rotating in the free stream flow) to provide initial power. The power supplied by windmilling may be supplemented with an APU-driven pneumatic starter. This arrangement may be less than optimal for at least the following reasons. First, the pneumatic starter adds weight to the aircraft. Second, the current trend in aircraft engine design is to increase both the bypass ratio and the overall pressure ratio of aircraft engines so as to increase engine efficiency. However, engines having higher bypass ratios and overall pressure ratios may be more difficult to start using the windmilling procedure described above.